The present invention relates to an electrically driven lift truck.
Electrically driven lift trucks usually have one or two electric motors for providing the tractive power to the truck and an electric motor for actuating a pump which supplies the hydraulic circuits of the truck. The motor or motors for the tractive power act on two front drive wheels of the lift truck and, more particularly, when two electric motors are provided each motor acts on a respective one of the two drive wheels. The hydraulic circuits connected to the pump actuated by the other electric motor include the hydraulic circuit for raising the forks and the hydraulic circuit for the power steering of the lift truck.
All these electric motors are dc motors and are supplied by batteries located on the lift truck.
As is well known, these dc electric motors, both those with permanent magnets and those with stator windings, have a commutator constituted by a ring of commutator segments and contact brushes for their operation. In use of the lift truck, the commutators of the electric motors are critical parts for various reasons.
First of all, the segments and the brushes are subject to mechanical wear so that their working life rarely exceeds 2000 hours in optimum conditions. This necessitates periodic stoppage of the lift truck and replacement of the commutator segments and brushes of the electric motors with consequent expense due to the inactivity of the truck and due to the maintenance itself.
Moreover a very high temperature develops in the commutator segments and this causes further problems.
Indeed it is necessary to cool the commutator segments by providing the electric motor with an internal cooling fan and by providing inlet and outlet apertures in the body of the motor for the air driven by the fan. The need to have these apertures prevents the internal parts of the motor from being insulated effectively and hence, when the lift truck operates in a dusty environment such as a cement factory, foundry, etc, the life of the commutator is reduced drastically compared with that indicated above for optimum working conditions of the lift truck.
It should also be noted that the temperature of the commutator segments cannot be checked directly by heat probes since it is a rotary part. Hence any overheating of the commutator segments is not detected and there is a possibility of malfunction or even seizing of the electric motor.
On the other hand, lift trucks with such dc motors have very satisfactory performances, particularly in terms of their ability to cope with an overload, controllability and speed of response.